An electrorheological fluid is a fluid whose visco-elasticity can be widely changed in a reversible manner by electrical control. Well known for the electrorheological fluid is the Winslow Effect; namely that certain fluids manifest an increase in apparent viscosity upon application of an electrical potential thereto. The old day electrorheological fluids which were typically composed of starch dispersed in mineral oil or lubricating oil were satisfactory for recognizing the importance of electrorheological effect, but lacked reproducibility.
In order to provide fluids having improved electrorheological effect and reproducibility, a number of proposals have been made with the main focus being on the powder used as the dispersed phase. There are known a variety of powders, for example, a highly water-absorbing resin having an acid group such as polyacrylic acid (Japanese Patent Application Kokai No. 93186/1978), an ion exchange resin (Japanese Patent Publication Kokai No. 31211/1985), and alumina silicate (Japanese Patent Application Kokai No. 95397/1987). All these electrorheological fluids are composed of a hydrophilic solid powder having water absorbed therein and being dispersed in an insulating oily medium. When a high electrical potential is externally applied to the fluid, the water helps the powder particles to polarize so that bridging occurs between the particles in a potential direction, resulting in a viscosity increase.
The hydrous electrorheological fluids based on such hydrous powder, however, suffered from many problems in practical applications. The problems included insufficient electrorheological effect over a wide temperature range, a limited service temperature range for avoiding evaporation and freezing of water, a marked current increase associated with a temperature rise, lack of stability due to water migration, and dissolution and corrosion of metal electrodes associated with a high electrical potential applied. It was thus quite difficult to use these hydrous electrorheological fluids in commercial applications.
In order to overcome the drawbacks of the hydrous electrorheological fluids, it was proposed to use powder of water-free particles in order to provide non-aqueous electrorheological fluids. A number of such non-aqueous fluids are known. For example, a fluid using a powder of uniform monophase particles, that is, particles of a uniform phase composed solely of an organic compound having electrical (or semiconductive) properties, such as organic semiconductor particles of poly(acene-quinone) or the like (see Japanese Patent Application Kokai No. 216202/1986 or GB 2 170 510 A published Aug. 6, 1986). Additionally, a fluid using a powder of thin film-coated composite particles, is known. The particles are covered with thin film layers having electrical (conductive/insulating) properties, so as to form dielectric particles in which organic or inorganic solid particles are coated on the surface with an electroconductive thin film layer and thereon with an electrically insulating thin film layer (see Japanese Patent Application Kokai Nos. 97694/1988 and 164823/1989).
Nevertheless, the non-aqueous electrorheological fluids, regardless of whether uniform monophase particles or thin film-coated composite particles are employed, have not been used in commercial applications because of the lack of long-term stability of their properties, poor reproducibility, an increased power consumption upon application of an electrical potential due to increased quantity of electric current flows across the fluid, and difficulty of industrial manufacture.
Therefore, there is a need for a powder suitable as the dispersed phase of a non-aqueous electrorheological fluid.
It is to be noted that in addition to the uniform monophase particles and thin film-coated composite particles mentioned above, several powders are known which have controlled electrical properties, such as, a carbon powder fired at different temperatures, a surface treated metal powder, and a metal coated inorganic powder. Since these powders were used mainly for their electrical properties, they had many problems including poor resistance against heat and oxidation and difficult control of electrical resistance and dielectric constant and thus were found to be of only limited application. Therefore, it is also desired to develop a powder having improved functions.